L.L. Anderson Dam Spillway Modification

To combat this impending threat, our team was commissioned by the Placer County Water Agency (PCWA) to design a master plan for widening and securing the Spillway. By adopting the existing Spillway as a cofferdam and applying precise rock stability evaluations through our team's extensive geotechnical exploration, our team cut the project down to two phases and saved the PCWA over $1.3M in excavation costs.

The 231-foot-high L.L. Anderson Dam (also known as the French Meadows Dam) is owned by the Placer County Water Agency (PCWA). The multi-purpose water supply and hydroelectric dam, originally constructed in 1966, diverts water from the Middle Fork American River watershed to the Rubicon River watershed to support power generation at a series of hydroelectric powerhouses, including the French Meadows Powerhouse. The LL Anderson dam spillway, located on the north (right) abutment of the dam, originally included a 48-foot-wide approach channel excavated in rock leading to a gated concrete ogee weir equipped with two 20-foot-wide by 18.5-foot-high radial gates.

The original probable maximum flood (PMF) estimate at the dam was estimated to have a peak inflow of 29,300 cfs. Hydraulic calculations showed that routing the original PMF through the reservoir and spillway resulted in a peak discharge of 12,200 cfs. However, since construction of the dam, the PMF was updated several times since the 1980’s, including the latest PMF inflow estimate of 59,100 cfs. Hydraulic routing of the latest PMF indicated that the spillway didn't have enough capacity and the dam would be overtopped by the flood, thus putting the dam at a high risk of failure under extreme flood conditions.